1. Field of the Invention
This invention relates to high frequency solid state transistors, and more particularly to Group-III nitride based high electron mobility transistors (HEMTs).
2. Description of the Related Art
HEMTs are a common type of solid state transistor that are regularly fabricated from semiconductor materials such as Silicon (Si) or Gallium Arsenide (GaAs). One disadvantage of Si is that it has low electron mobility (approximately 1450 cm2/V-s), which produces a high source resistance. This resistance can degrade the Si based HEMT's high performance gain. [CRC Press, The Electrical Engineering Handbook, Second Edition, Dorf, p.994, (1997)]
GaAs based HEMTs have become the standard for signal amplification in civil and military radar, handset cellular, and satellite communications. GaAs has a higher electron mobility (approximately 6000 cm2/V-s) and a lower source resistance than Si, which allows GaAs based devices to function at higher frequencies. However, GaAs has a relatively small bandgap (1.42 eV at room temperature) and relatively small breakdown voltage, which prevents GaAs based HEMTs from providing high power at high frequencies.
Improvements in the manufacturing of AlGaN/GaN semiconductor materials have focused interest on the development of AlGaN/GaN HEMTs for high frequency, high temperature and high power applications. AlGaN/GaN has large bandgaps, high peak and saturation electron velocity values [B. Belmont, K. Kim and M. Shur, J.Appl.Phys. 74, 1818 (1993)]. AlGaN/GaN HEMTs can also have 2DEG sheet densities in excess of 1013/cm2 and relatively high electron mobility (up to 2019 cm2/Vs) [R. Gaska, J. W. Yang, A. Osinsky, Q. Chen, M. A. Khan, A. O. Orlov, G. L. Snider and M. S. Shur, Appl.Phys.Lett., 72, 707 (1998)]. These characteristics allow AlGaN/GaN HEMTs to provide high power at higher frequencies.
AlGaN/GaN HEMTs have been grown on sapphire substrates and have shown a power density of 4.6 W/mm and a total power of 7.6 W [Y. F. Wu et al., IEICE Trans.Electron., E-82-C, 1895 (1999)]. More recently, AlGaN/GaN HEMTs grown on SiC have shown a power density of 9.8 W/mm at 8 GHz [Y. F. Wu, D. Kapolnek, J. P. Ibbetson, P. Parikh, B. P. Keller and U. K. Mishra, IEEE Trans.Electron.Dev., 48, 586 (2001)] and a total output power of 22.9 at 9 GHz [M. Micovic, A Kurdoghlian, P. Janke, P. Hashimoto, D. W. S. Wong, J. S. Moon, L. McCray and C. Nguyen, IEEE Trans.Electron.Dev., 48, 591 (2001)].
U.S. Pat. No. 5,192,987 to Khan et al. discloses GaN/AlGaN based HEMTs grown on a buffer and a substrate. Other AlGaN/GaN HEMTs and field effect transistors (FETs) have been described by Gaska et al., “High-Temperature Performance of AlGaN/GaN HFET's on SiC Substrates,” IEEE Electron Device Letters, Vol. 18, No 10, October 1997, Page 492; and Ping et al., “DC and Microwave Performance of High Current AlGaN Heterostructure Field Effect Transistors Grown on P-type SiC Substrates,” IEEE Electron Devices Letters, Vol. 19, No. 2, February 1998, Page 54. Some of these devices have shown a gain-bandwidth product (fT) as high as 67 gigahertz [K. Chu et al. WOCSEMMAD, Monterey, Calif. (February 1998)] and high power densities up to 2.84 W/mm at 10 GHz [G. Sullivan et al., “High Power 10-GHz Operation of AlGaN HFET's in Insulating SiC,” IEEE Electron Device Letters, Vol. 19, No. 6, Page 198 (June 1998); and Wu et al., IEEE Electron Device Letters, Volume 19, No. 2, Page 50 (February 1998)].
FIG. 1, shows a typical AlGaN/GaN HEMT 10 that comprises a GaN buffer layer 11 adjacent to a sapphire or silicon carbide substrate 12 and a AlxGa1−xN (x˜0.1-0.5) layer 13 adjacent to the GaN buffer layer 11, opposite the substrate 12. A nucleation layer 14 can be included between the substrate 12 and the GaN buffer layer 11, to reduce the lattice mismatch between the two layers. The HEMT 10 also includes source, gate and drain contacts 15, 16, 17. The Al content in the AlxGa1−xN creates a piezoelectric charge that accumulates at the interface with the GaN layer to form a two-dimensional electron gas (2DEG) 18. As the Al content in the AlxGa1−xN layer is increased, the piezoelectric charge is also increased, which results in a beneficial increase the HEMT's 2DEG and transconductance.
However, the 2DEG's mobility is generally limited by the interface roughness and piezoelectric scattering at the interface between the GaN and AlxGa1−xN layers 11, 13, which are the result of localized randomness in the AlxGa1−xN layer 13 near the interface.
By substituting an AlyGa1−yN (y=1 or y˜1) layer for the AlxGa1−xN (x˜0.1-0.5) layer 13, certain advantages can be achieved. The 2.4% lattice mismatch between AlN (AlyGa1−yN for y=1) and GaN results in the maximum possible piezoelectric charge at the interface between the two layers. Using an AlN layer also reduces the piezoelectric scattering between the layers that can limit the 2DEG mobility.
However, the high lattice mismatch between AlN and GaN dictates that the thickness of the AlN layer should be less than 50 Å. If the layer is thicker, the device can experience problems with its ohmic contacts, the material quality in the layer begins to degrade, the device's reliability decreases, and the material is more difficult to grow. However, a HEMT with a 50 Å or less AlN layer is susceptible to high gate leakage.